The dampener disclosed in the aforementioned U.S. Pat. No. 4,555,938 has achieved wide acceptance, and has undergone evolvement during the past decade (as demonstrated in U.S. Pat. No. 5,134,935). However, it retains the essence of the direct continuous contact dampening process, in which the dampener form roller runs in continuous contact with the press form cylinder and the form (printing plate) thereon, and in which the metering roller runs in continuous pressure contact with the form roller, defining a downwardly and inwardly moving nip between the rollers.
In these dampeners the surface of the form roller is made of a compressible material while the surface of the standard metering roller is much more rigid. This standard metering roller is somewhat longer than the form roller and has developed to include a steel core, a plastic sleeve pressed onto the core, hard end collars that cap each end of the sleeve and end nuts securing the end collars in place. The sleeve diameter must be cut to within .+-.0.003 inch of the end collar diameters to ensure a relatively smooth transition between the surfaces. A pair of seal members are urged into contacting relationship with the radial end faces of the form roller and the circumference of each standard metering roller end collar. These seals, together with the nip and surface of each roller immediately above the nip, define a reservoir area into which dampening solution is supplied, the resulting pool being kept at a predetermined depth.
No attempt is made to prevent or minimize the transfer of ink from the inked plate onto the surfaces of the dampening form roller and, therethrough, the dampening metering roller. Indeed, having ink on both the form and metering roller is necessary for the dampening system to function properly. This is because the inked rollers facilitate the formation of the ink/fountain-solution emulsion in the reservoir area.
During printing, the emulsion is milled as it passes through the nip of the two rollers and passes, via the lower surface of the form roller, to the plate--the fountain solution portion of the emulsion wetting the hydrophilic non-image areas of the plate, the ink portion of the emulsion adhering the hydrophobic/oliophilic image areas of the plate.
If, for example, the metering roller working surface is to become hydrophilic for any reason--and thereby not carry any ink--the emulsion will not form properly in the reservoir area and the dampening system will tend to carry too much fountain solution to the plate. This condition leads to printing-quality problems such as washed out images and paper-feed problems caused by curled stock.
During operation of the dampening system the seals slowly wear, particularly the surfaces in contact with the ends of the form roller, and tend to move longitudinally inward toward the juncture of the end collars and the plastic sleeve. If this condition is not timely detected, the seals gradually move inward of the end collars and onto the working surface of the plastic sleeve. At this point, heat builds up and causes the plastic material to melt and form a groove. Once formed, the groove passes excessive amounts of fountain solution to the plate, causing poor print quality, paper curl, etc.
Damage to the ends of the plastic portion of the metering roller could be avoided if the full surface of the metering roller were made of a material similar to the wear resistant end collars. But, in order for the dampener to function properly, the roller would have to have surface characteristics similar to that of the plastic sleeve of the standard metering roller--i.e., hydrophobic.
In addition, because of the relatively soft nature of the plastic sleeve, it is susceptible to nicks and gouges when contacted by tools during adjustment of the dampener or when dropped on a hard surface during handling.
Various metering roller forms and finishes have been experimented with, but without success. For example, metering rollers coated entirely with ceramic, similar o that disclosed by Yokoyama et al, in U.S. Pat. No. 4,991,501, have been tried. But, because of the hydrophilic nature of finished plain ceramic, the needed ink/fountain-solution emulsion could not properly form, causing excessive amounts of fountain solution to be transferred to the plate. Metering rollers have also been produced from various types of plastic compounds, but, because the wear characteristics were poor, have also fail prematurely.